This invention relates to a video display panel driven with relatively high voltage and more particularly to a display using a capacitive flat matrix display panel as an electroluminescent panel (called hereinafter "thin film EL panel").
A typical example of capacitive flat matrix display panels is a thin film EL panel and the following description is directed toward the thin film EL panel for the sake of illustration only.
As disclosed and illustrated in U.S. Pat. No. 3,975,661, DRIVING METHOD FOR A THIN-FILM ELECTROLUMINESCENT ELEMENT OF A THREE-LAYER CONSTRUCTION, assigned to the same assignee as this application, a representative thin film EL display panel has a three-layered structure shown in a perspective view partly in cross section in FIG. 1, the disclosure of which is incorporated herein. A predetermined number of transparent electrode strips 2 are disposed on a glass support 1. Further, a layer 3 of dielectric material such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 and Al.sub.2 O.sub.3, a layer 4 of electroluminescent material, for example ZnS doped with Mn (yellowish orange light) and a second layer 3 of dielectric material such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2, Al.sub.2 O.sub.3 are disposed by a well known thin film technique such as vacuum deposition and sputtering, each having a thickness ranging from 500 to 10000 .ANG.. This results in a double-isolation three-layered structure of the EL display panel. A different family of strip electrodes 5 is disposed in a direction normal to the transparent electrodes 2 to form an electrode matrix array together with the transparent electrodes. With such a three-layered thin film EL display panel, if one of the electrodes 2 in the first family and one of the electrodes 5 in the second family are selected, the minute area where the selected ones of the electrodes cross will emit light. This corresponds to a picture element of an image like a character, a symbol or a pattern to be displayed. The above described type of the EL panel is more attractive than the prior art dispersed powder type EL panel from standpoints of light intensity, working life and performance stability.
For displaying by use of the thin film EL panel, an individual drive circuit is needed for each of the respective electrodes whatever character, graph or image is desired to appear on the panel. The use of discrete elements in the buildup of the drive circuits attaches an increased number of components, complicated interconnections between the components and thereby noise. The number of circuit nodes to be adjusted is increased and fluctuations in the operating properties of the discrete elements would cause undesirable varied and uneven image quality. In order to display continuously varying and half-tone images like TV images, it is necessary to convert such analog signals as TV transmission signals into digital signals compatible with a drive circuit for the thin film EL panel. Mainly because thin film EL panel requires a considerably high light emitting voltage, drive elements must be high breakdown voltage elements. The prior art drive circuit employs as MOS integrated circuit in which gate voltage is converted into drain current for modulation purposes. However, such an IC suffers from the disadvantages that deviations in mutual conductance gm and gate-to-source threshold voltage are prejudicial to modulation operation.